Windows:
We already use this API in Firefox, should be easy to surface the info

https://social.msdn.microsoft.com/Forums/vstudio/en-US/085ea547-b376-48a7-83b2-2480cf6b500e/getting-the-dns-suffixes-in-c-without-wmi?forum=vcgeneral
https://docs.microsoft.com/en-us/windows/desktop/api/iphlpapi/nf-iphlpapi-getadaptersaddresses
printf("\tDNS Suffix: %wS\n", pCurrAddresses->DnsSuffix);

Linux:
We call res_ninit already. We need to extract the info from the struct, not clear how well it works

https://stackoverflow.com/questions/12227438/retrieving-dhcp-allocated-domain-suffix-in-linux-c

• https://fossies.org/dox/libbind-6.0/resolv_8h_source.html

OSX:
We don't use this API, it's unclear how we'd use it. Linux method might work as well.

https://developer.apple.com/documentation/networkextension/nednssettings/1406658-searchdomains#see-also

There is also a Windows Registry way of getting the DNS suffix list:
https://computerstepbystep.com/dns-suffix-search-list.html

Dropping this info from Adam Roach:

Linux puts the values in `/etc/resolv.conf` on a line labeled "search", if configured.

OS X does as well, although that file has historically done a poor job of tracking dynamic changes to network configuration. You can also get it by running `networksetup -getsearchdomains <service>` (see `networksetup -listallnetworkservices` for the list of services), which tells me there has to be some underlying API for retrieving it.

Although we may use this bug to implement the windows version, and spin off other bugs for the rest of the platforms.

https://hg.mozilla.org/mozilla-central/rev/71e49603ed69